Selective hydrogenation of carbazole



Unite States Patent 2,999,097 SELECTIVE HYDROGENATION 0F CARBAZOLE Hans Dressler, Pitcairn, and Joseph C. Martini, Verona,

Pa., assignors to Koppers Company, Inc., a corporation of Delaware No Drawing. Filed Oct. 2, 1959, Ser. No. 843,923 Claims. (Cl. 2611-315) This invention relates to the hydrogenation of carbazole. In one specific aspect, it relates to a method of selectively hydrogenating carbazole to produce in high yield 1,2,3,4-tetrahydrocarbazole.

Carbazole occurs in coal tar to the extent of 2-3% in the anthracene oil fraction thereof. It is well known that carbazole can be hydrogenated either partially or completely to provide tetrahydrocarbazole, hexahydrocarbazole and dodecahydrocarbazole. These interesting compounds are useful as, inter alia, starting materials for preparing a number of valuable carbazole derivatives. For example, tetrahydrocarbazole can be reacted, as described in the copending application of Hans Dressler, S. No. 837,588 filed September 2, 1959, with 2- and 4-vinyl pyridines to provide novel compounds which elfectively inhibit the growth of wood-destroying fungi.

The known processes for making tetrahydrocarbazole by the catalytic hydrogenation of carbazole are expensive to employ and they provide relatively low yields of the desired product. The best known methods are those described by H. Adkins and H. L. Coonradt, J. Am. Chem. Soc. 63, 1563 (1941). By one of their methods, purified carbazole is hydrogenated using a Raney nickel catalyst in pure dioxane at 230 C. at a pressure of 3500-4400 p.s.i.g. of hydrogen. The yield of tetrahydrocarbazole thus obtained is only 33%. A somewhat higher yield (72%) is obtained by substituting a copper chromite catalyst for the Raney nickel under the same conditions of temperature and pressure.

Quite surprisingly, We have discovered a novel method of making 1,2,3,4-tetrahydrocarbazole which requires no pre-purification of the carbazole and solvent, no organic medium, and considerably lower operating pressures than those taught by the prior art. Using our new method, hydrogenation stops essentially at the tetrahydrocarbazole stage; thus the desired product is obtained in yields of 85% or higher and it is substantially free of the more completely hydrogenated carbazoles.

It is, therefore, an object of the present invention to provide a novel and more economical method for ob taining the useful tetrahydrocarbazole in yields higher than those obtainable by heretofore known methods.

In accordance with the invention, carbazole is reacted with hydrogen in an aqueous medium having a pH of at least about 12 in the presence of a nickel catalyst supported on an inert carrier at a temperature of about 200-260 C. and at a total pressure of at least about 800 p.s.i.g.

In contrast with the previously known processes, technical grade carbazole (which is generally about 97% pure) is suitable as a starting material in the invention. Heretofore, an extensive pre-purification procedure was required, which involved (1) recrystallization from dioxane, (2) two refluxes with a nickel catalyst in dioxane, and (3) agitation for two hours at 60 C. under hydrogen pressure.

The nickel catalyst suitable for the invention is metallic (reduced) nickel supported on an inert carrier such as kieselguhr, fullers earth, kaolin, activated carbon, silica gel, alumina, and silica-alumina. Such catalysts contain from about 70% nickel based upon the total weight of the composition. The catalyst is employed in an amount ranging between about 130% nickel based upon the weight of the carbazole.

2,999,097 P atented Sept. 5, 1961 genation takes place is of particular importance. We have found that a pH of at least about 11 is necessary in order to provide high yields of tetrahydrocarbazole. If hydrogenation is conducted at a pH of 5.5, the yield of tetrahydrocarbazole is only 46%; at a pH of 10 the yield is 67%. The pH of the medium is conveniently adjusted by adding a strong base, preferably an alkali metal hydroxide such as potassium or sodium hydroxide. By adjusting the pH to within the range of 11-14, we are able to obtain yields of tetrahydrocarbazole of about and higher.

The temperature of the reaction is maintained between about 200 and 260 C. Below about 200 C. reaction time is unduly prolonged and there is a measurable decrease in the degree of selectivity obtained. The loss in selectivity results in a loss of yield and the formation of measurable quantities of hexahydrocarbazole and dodecahydrocarbazole in the final react-ion mixture. Thus, a separation problem is incurred and .the overall economy of the process is adversely efiected. Above about 260 C. decomposition becomes noticeable and as a result low yields of the desired tetrabydrocarbazole are obtained.

The total pressure of the system varies with the temperature used for the reaction. The total pressure, which includes the autogenous pressure resulting from maintaining the system at the desired temperature level and the partial hydrogen pressure under which the systemis placed, should be at least about 800 p.s.i.g. and can be varied conveniently between 800 and 1200 p.s.i.g. No particular advantage is seen in using higher pressures. The hydrogen pressure applied to the system ranges preferably between about and 400 p.s.i.g. It should be at least 100 p.s.i.g and no advantage is obtained by the use of pressures higher than 400 p.s'.i.g.

The reaction time is that required for the absorption of the theoretical quantity of hydrogen; i.e. 2 moles of hydrogen per mole of carbazole. Generally, this takes place in about /z2 hours. Because of the high degree of selectivity of the process, hydrogen absorption virtually stops after the carbazole is hydrogenated to the tetrahydrocarbazole stage. Hydrogen absorption is measured by a pressure drop within the reactor. The reactor can be intermittently repressured with hydrogen to maintain the required hydrogen pressure until the pressure of the system remains substantially constant.

After the reaction is completed, the reaction mixture is allowed to cool and is thereafter filtered. The in solubles are conveniently extracted using a water-immiscible organic solvent including aromatics such as benzene, toluene, xylene; higher boiling ketones, ethers, and the like. 1,2,3,4-tetrahydrocarbazole can be recovered from the solvent by distillation or it can be separated from the solvent and the residual unreacted carbazole by a further extraction with a strong mineral acid, preferably hydrochloric acid of a concentration of 2537%. Upon dilution of the acid extract with water 1,2,3,4- tetrahydrocarbazole precipitates in relatively pure form and the unreacted carbazole remains in the solvent layer. The product thus obtained can be recrystallized if desired.

Our invention is further illustrated by the following examples.

Example I A 1 gallon stainless steel autoclave was charged with 167 g. (1.0 m.) of carbazole, 1000 ml. of water adjusted to pH 12 with dilute potassium hydroxide, and 85 g. of a pre-reduced nickel-on-kieselguhr (55% by weight nickel) catalyst. The mixture was stirred and heated to 250 C., at which temperature a pressure of 680 p.s.i.g.

was reached. The autoclave was then pressured to a total pressure of 1000 p.s.i.g. with hydrogen and a fast reaction ensued. When the total pressure fell to 800 p.s.i., the autoclave was repressured with hydrogen to 1000 p.s.i.g. In 60 minutes, the hydrogen absorption practically stopped. The mixture was allowed to cool, the autoclave was vented, and the catalyzate was filtered. The insolubles were extracted with a 500 ml. and a 200 ml. portion of benzene, and the combined benzene extract was shaken with three 200 m1. portions of 30% hydrochloric acid, in which carbazole is insoluble. The acid extract was then diluted with water to 15-20% concentration to give as a precipitate 1,2,3,4-tetrahydrocarbazole. The yield of tetrahydrocarbazole thus obtained was 134 g. (87% of the theory). The product had a melting point of 115-118 C. After one recrystallization from ethanol, the melting point was 118-9 C.

Example 11 The autoclave was charged with 167 g. (1.0 m.) of carbazole, 1000 ml. of distilled water (pH 5.5), and 85 g. of the nickel catalyst, and hydrogenated as in Example I. The yield of tetrahydrocarbazole was 46% of the theory.

Example Ill 1. Method of making tetrahydrocarbazole comprising 4 contacting carbazole with hydrogen in an aqueous medium having a pH of at least about 11 in the presence of a nickel catalyst supported on an inert carrier at a temperature of 200-260 C. under a hydrogen partial pressure of about -400 p.s.i.g.

2. Method of making tetrahydrocarbazole comprising contacting carbazole with hydrogen in an aqueous medium having a pH of about 11-14 in the presence of a nickel catalyst supported on an inert carrier consisting essentially of 1070% by weight nickel at a temperature of 200-260 C. under a total pressure of about 800- 1200 p.s.i.g.

3. Method of making tetrahydrocarbazole comprising contacting carbazole with hydrogen in an aqueous medium having a pH of about 11-14 in the presence of a catalytic amount of nickel catalyst supported on kieselguhr at a temperature of 200-260 C. under a hydrogen partial pressure of about 100-400 p.s.i.g.

4. Method of making tetrahydrocarbazole comprising contacting carbazole with hydrogen in an aqueous medium having a pH of about 11-14 in the presence of a nickel catalyst supported on kieselguhr in an amount of about 1-30% metallic nickel based on the weight of carbazole at a temperature of 200-260 C. under a total pressure of about 800-1200 p.s.i.g.

5. Method of making tetrahydrocarbazole comprising contacting carbazole with hydrogen in an aqueous medium having a pH of at least about 11 in the presence of a catalytic amount of a nickel catalyst supported on an inert carrier at a temperature of 200-260 C. under a hydrogen partial pressure of about 100-400 p.s.i.g.

References Cited in the file of this patent FOREIGN PATENTS Germany Dec. 11, 1930 OTHER REFERENCES 

1. METHOD OF MAKING TETRAHYDROCARBAZOLE COMPRISING CONTACTING CARBAZOLE WITH HYDROGEN IN AN AQUEOUS MEDIUM HAVING A PH OF AT LEAST ABOUT 11 IN THE PRESENCE OF A NICKEL CATALYST SUPPORTED ON AN INERT CARRIER AT A TEMPERATURE OF 200-260*C. UNDER A HYDROGEN PARTIAL PRESSURE OF ABOUT 100-400 P.S.I.G. 